This invention relates to a rotation control device for a rotary head in a rotary head type magnetic recording and reproducing device such as an R-DAT (rotary head type digital audio tape recorder) and, more particularly, to improvement in response characteristics in a rotation phase control in such rotary head.
The R-DAT is a device which converts analog signals such as an audio signal into PCM signals, records the PCM signals on a magnetic tape and reproducing the same.
Rotation control of a rotary head in an R-DAT is generally made by combining speed control and phase control. The speed control is a control in which rotation of a rotary drum is servo controlled to a predetermined revolution number (normally 2000 rpm in the case of reproduction and recording in the mode I) to maintain a relative speed between the rotary head and a magnetic tape at a predetermined value (e.g., 3.13 m/sec in mode I). The phase control is a control in which phase of rotation of the rotary head is controlled to synchronize the position of the head with a recorded signal so that recorded signals for one track will be correctly recorded on one track (if there occurs a phase error, signals are sometimes recorded from the midway of the track).
The phase control is normally performed also during reproduction of a signal. In a reproduction system from the head to the signal processing circuit, a reproduced clock obtained from a reproduced signal is used whereas in a digital-to-analog converter provided on the output side, refererence clock such as Xtal is generally used and, accordingly, if the reference signal for rotation of the head drum is not synchronized with this reference clock, excess or shortage of data will occur through the system.
A prior art rotation control device for a rotary head is shown in FIG. 2. The rotary head 2 includes an FG (frequency generator) 30 which outputs a signal at a period determined by equally dividing one rotation of a drum 1 and a PG (phase generator or pulse generator) 32 which detects rotation reference position of the drum 1 (e.g., rotation position of the drum 1 when a head A, for example, has reached the track start position at the lower end of the tape).
Since the frequency of an FG detection signal from the FG 30 is proportional to the rotation speed of the drum 1, a speed servo circuit 12 obtains the rotation speed of the drum 1 from the FG detection signal from the FG 30 and obtains a speed error between the detected rotation speed and an ordered speed. A PG detection signal from the PG 32 represents the rotation reference position of the drum 1 and a phase servo circuit 14 compares the PG detection signal with a PG reference signal in phase for obtaining a phase error .theta..
The speed error signal and the phase error signal are added together by an adder 16 and used for controlling rotation of a drum motor 28 through a drive amplifier 18 so as to reduce these errors to zero.
In the prior art rotation control, the phase error .theta. is detected only once for one rotation of the drum 1. The phase control in the prior art rotation control therefore is slow in response speed and it takes time before reaching servo lock. Further, in a case where the revolution number of the drum 1 is 2000 rpm, sampling frequency of the phase error .theta. is a low frequency of 33.3 Hz (=2000 rpm/60 sec.) and, accordingly, upper limit response frequency is not more than 16 Hz even if there is no delay element (if a delay element such as phase delay in AFC closed loop is considered, the upper limit response frequency will be below 10 Hz) so that response to phase variation exceeding this frequency cannot be expected.
It is, therefore, an object of the invention to provide a rotation control device for a rotary head which has improved its response characteristics (response speed, upper limit response frequency etc.).